Many commercially available plywood panels are manufactured from wood veneers and have phenol formaldehyde resins used as the primary adhesive between the veneers. The plywood panels manufactured by this process are inexpensive to produce, but are prone to impact damage from dynamic loading, particularly at the edge of the panel.
Various adhesives have been used in an effort to improve the impact strength of plywood panels and other wood composite materials. See, e.g., U.S. Pat. No. 3,493,417 to Moren et al. which discloses an adhesive composition comprising a polyol compound, a polymerizable amineformaldehyde resin and a curing catalyst for that resin; U.S. Pat. No. 3,563,788 to Brown et al. which discloses an aqueous phenol-formaldehyde resole resin solution; U.S. Pat. No. 4,215,173 to Hubbard which discloses an alphacyanlacrylate as a bonding resin; U.S. Pat. No. 4,879,160 to Knudson et al. which discloses an isocyanate type resin adhesive; and U.S. Pat. No. 4,464,510 to Czvikovszky et al. which discloses an unsaturated polyester resin and a vinyl monomer.
In addition, various thermoplastic resins, such as polyesters and polyolefins, have been used as adhesives for laminates including between wood composite layers. See, e.g., U.S. Pat. No. 4,865,912 to Mitsumata; U.S. Pat. No. 3,817,822 to Dawes which discloses at least two plies of wood sheet bonded together with a polyethylene film cross-linked with an organic peroxide; U.S. Pat. Nos. 4,407,689 to Ohtsukai, 4,753,832 to Brown et al. and 4,948,640 to Givens et al., all of which disclose the use of a polyethylene based adhesive; U.S. Pat. Nos. 4,335,188 to Igi et al. and 4,318,837 to Streets et al., both of which disclose adhesive mixtures containing polyethylene terephthalate; and U.S. Pat. No. 4,062,907 to Sublett which discloses a modified polyester or blend which may contain polyethylene terephthalate.
The use of thermoplastic resins as adhesives for wood composites are desirable from the standpoint of providing good adhesive strength, but they normally increase the cost of manufacture of the composites. Additionally, the high melt temperature of some thermoplastic resins, e.g., polyethylene terephthalate which has a melt temperature of 260.degree. C., as compared to the burn temperature of wood, e.g., 220.degree. C., has resulted in the infrequent use of polyester resins as adhesives for wood composites.